Alternating current (AC) magnetic tracking systems typically use a method in which magnetic fields are generated and measured using a system of orthogonal coils to generate position and orientation information which is used by a computer system. One common application is tracking a user for the purpose of playing a video game or being immersed in a virtual reality world.
In some prior art embodiments, a transmitter has three orthogonal coils which generate the magnetic field signal, and may be located in a fixed position, such as in a local base station, video game console or other apparatus. A receiver in a tracking device similarly has three orthogonal magnetic field sensing coils, and provides information about the sensed magnetic field to a processor, which derives the position and orientation of the tracking device relative to the base station from such information.
There are a number of factors that limit the accuracy of such magnetic tracking systems. The strength of the magnetic field between a tracking device and a base station transmitter suffers from attenuation over distance, which decreases accuracy in the detection of the magnetic field by the tracker. Objects located between the tracking device and the base station may cause distortion in the magnetic field, particularly if those objects contain ferrous materials. Conducting surfaces in the environment may also contain eddy currents induced by the generated magnetic field which in turn generate secondary magnetic fields that interfere with the ability of sensors to detect the source magnetic field.
Another factor is that errors in detection of the magnetic field by a receiver which cause the position and orientation of the receiver to be incorrectly determined are typically larger the farther away the receiver is from the transmitter. Further, the error may not be linear, i.e., the error may increase faster than the distance between the receiver and the transmitter.
Still another source of ambiguity in wireless magnetic tracking systems is the magnetic field signal phases that are received by the sensor. In wired tracking systems, a synchronization (“sync”) signal between the transmitter and receiver indicates the phases of the magnetic field signals and enables the receiver to extract the relative signs of the signals. (In some cases another means may be used to pass this information.) The signs indicate the orientation and direction of the magnetic field (i.e. positive or negative in direction and its orientation).
In a wireless system, when there is no sync signal between the receiver and transmitter, there is an ambiguity with respect to the signal's signs and thus also with respect to the position and orientation of the receiver relative to the transmitter. When the phase of the transmitted signal is unknown, the sign of the received signals is ambiguous, which can create an additional uncertainty in the location of the receiver with respect to its direction from the transmitter.
Changes in the magnetic field, error in the determination of the magnetic field and phase ambiguity all contribute to inaccurate determination of the position and orientation of a receiver relative to a transmitter in a magnetic tracking system.